1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems in which a plurality of elevator cars are controlled to answer certain floor calls according to predetermined strategies.
2. Description of the Prior Art
Various control arrangements have been used in an attempt to provide more equitable elevator service to all floors of a building during the various traffic conditions encountered throughout a typical day. Elevator cars have been assigned to specific hall calls, or groups of hall calls, with the priority in assigning a specific call to a car being based on the order or registry of the call. Thus, the relative waiting times of the calls establishes the priority, rather an absolute time value. U.S. Pat. Nos. 2,104,478 and 2,104,522 assign an elevator car to a predetermined number of hall calls, with the calls assigned being based on the order of registry. U.S. Pat. No. 3,371,747 selects a predetermined number of hall calls, by order of registration, as priority calls, and assigns elevator cars thereto. U.S. Pat. No. 3,561,571 gives certain cars a special status, and these special status cars respond only to a call having a high priority, based on the order of registration. U.S. Pat. No. 3,645,361 selects the longest wait call for priority, based on order of registration.
In addition to keeping track of call registration order, timers have also been used to time the registration time of hall calls in order to determine when calls should receive certain predetermined priority treatment. For example, elevator cars which are busy answering calls for elevator service may treat hall calls differently as a function of the total time registered and the specific condition of the elevator car. In U.S. Pat. No. 2,347,054, an elevator car will stop for a short term call if it is on schedule and has not already accepted a predetermined number of hall calls to answer, and as it falls behind schedule and/or accepts more hall calls, it will stop for only calls registered for a medium, or a long term, depending upon how far behind schedule and/or how many calls it has accepted. In U.S. Pat. No. 2,624,425 the duration of a hall call necessary to stop a car varies as a function of car load. In U.S. Pat. No. 3,256,958, zones of one or more floors are established, and if there are more down zones requiring service then down running cars in position to serve the calls, a demand is created for the zone of the call, to which a car is assigned when one becomes available. If the demand exists for a predetermined period of time, it becomes a higher priority demand to which a car will be assigned regardless of the number of down calls and down running cars.
U.S. Pat. No. 3,256,958 further assigns available cars to a plurality of down demands, none of which are timed out, by assigning the first car to the highest down demand, and, until this highest down demand call is canceled, subsequent cars are assigned to the demand which represents the midpoint of the remaining unassigned down zone demands. Should a down zone demand become timed out, it receives preference over non-timed out demands, and if there are more than one timed out demand, the highest timed out demand is assigned first.
British Patent 931,301 times the calls in down zones, and sends available cars to zones and/or calls in the zone, on the basis of total lengths of time the zone demand or floor call has been registered. U.S. Pat. No. 3,506,094 selects the oldest call for priority service. Inhibiting all the call timers should one reach a predetermined maximum value, to retain the relative priorities of the calls.